As well known, for detecting mechanical features of a metal material, hardness tests exist that are executed on samples by an element having high hardness (indenter) that is pushed, with a controlled contact force P, against a surface of the sample for causing a permanent deformation on it.
For describing satisfactorily the mechanical features of a metal material, the following parameters are used: yield stress σsn, strain-hardening coefficient n and elastic modulus E. In fact, the strain-stress curve σ-ε, which describes graphically the features of a metal material, is completely definable on the basis of such amount, for example according to the known Hollomon equation:
  σ  =      {                                        E            ·            ɛ                                                ɛ            ≤                                                            σ                  sn                                E                            .                                                                                      σ              sn                              (                                  1                  -                  n                                )                                      ·                          E              n                        ·                          ɛ              n                                                            ɛ            >                                          σ                sn                            E                                          
In addition to the elastic modulus, which is a known starting parameter for a material or in any case a parameter obtainable with non-destructive tests, the other two parameters, i.e. the yield stress σsn and the strain-hardening coefficient n, are obtained from the so called tensile test.
In known indentation apparatus, normally used for carrying out measurements of hardness of materials, the motion of the indenter towards the sample is effected, for example, by a step motor. The motor generates a force F that is amplified mechanically and then transmitted to the indenter through a suitable kinematical chain. The amplification of the force aims at a precise control on the stroke of the indenter and, in particular, on the depth of penetration h of the indenter in the material. This is possible, since, during the amplification, the force generated is multiplied for a fixed factor and at the same time the movement is reduced for the same factor. As above said, in the apparatus of prior art, the amplification of the force is obtained in a mechanical way, i.e. combining a reduction gear to the motor. The mechanical amplification allows then a precise measurement of the stroke h of the indenter and, however, it causes a high inertia and problems of mechanical wear.
Furthermore, to ensure to the apparatus a high centering precision during an indentation test, it is necessary to assemble the indenter on a structure having high structural stiffness, generally equipped with guides on which the indenter is mounted for driving it with respect to the tested sample. Therefore, the apparatus of prior art are structurally complex and have a high encumbrance.